The two principal steps of crystallization are the formation of crystal seeds (nucleation) and crystal growth. In most industrial processes, crystallization is primarily based on crystal growth, not nucleation.
The state of the art with respect to crystallization is represented, for example, by Mathlouthi, M. and Reiser, P. (ed.), Sucrose, Properties and Applications, Blackie Academic & Professional, Suffolk, Great Britain, 1995, p. 49 ff. This disclosure explains the crystallization mechanism with respect to both nucleation and crystal growth. With regard to the industrial crystallization of sucrose, this publication states, for instance, that one must avoid concentrating the solution up to the nucleation zone, i.e. the zone whereat spontaneous nuclei formation readily occurs (p. 58); one must avoid the creation of an uncontrolled number of seeds (p. 59); the crystals to mother liquor ratio cannot be increased beyond a certain value (pp. 59-60); and crystallization should be carried out in the metastable zone not too close to the nucleation zone and the saturation curve (pp. 60-61, and pp. 63-64). The metastable zone is the zone where spontaneous crystal formation will occur only if crystals are present. It is emphasized that in this zone no new crystals are formed in the absence of seeds. Furthermore, according to this publication (cf. e.g. pp. 57 and 58) impurities decrease the crystal growth rate and may even block the growth completely.
With an increase in supersaturation and a decrease in the temperature of the solution, the viscosity of the solution also increases, thus slowing down and eventually completely blocking diffusion of the molecules through the liquid layer surrounding the crystals or crystal nuclei to the crystal surface, and hence prior art processes based on crystal growth are no longer possible. In accordance with the prior art, high viscosity has also been regarded as a downright impediment to the separation of crystals from the mother liquor.
In respect to sucrose crystallization, these problems have also been discussed in the above publication Mathlouthi, M. and Reiser, P. (ed.), Sucrose, Properties and Applications.
The methods for recovering sucrose employed in the sugar industry typically comprise three successive crystallization steps. In the last step, which is known as `c` crystallization, the sucrose content of the starting material syrup is about 73-75% on dry substance; this crystallization method is slow and difficult, and yet the sucrose purity (% of sucrose on dry substance) of the run-off, i.e. molasses, obtained therefrom is still typically as high as about 58%. There are several methods by which it is sought to improve the sucrose yield, i.e. to diminish the sucrose purity of molasses. Such methods include the Quentin and Steffen methods and the molasses fractionation methods of the type described in Finnish Patent 77 845 (Suomen Sokeri Oy; Heikkila, Melaja, Millner, Virtanen; corresponds to international published application WO 81/02420). Such fractionation methods enable a fraction enriched with betaine wherefrom betaine can be recovered to be obtained from molasses.
In conventional methods, it has only been possible to crystallize xylose if the xylose purity has been at least about 70% by weight on dry substance. In that connection, it has been necessary to first purify the xylose-containing solution obtained, for example, as a result of hydrolysis of vegetable-derived material to the required degree of purity by various ultrafiltration, ion exchange, decolouring, ion exclusion or chromatographic separation methods or combinations of these; furthermore, auxiliary solvents diminishing the solubility of xylose have been employed to crystallize xylose.
The above xylose separation, purification and crystallization methods have been described, for instance, in U.S. Pat. No. 4,631,129 (Heikkila, H.; Suomen Sokeri Oy), U.S. PAt. No. 4,075,406 (Melaja, A. J. & Hamalainen, L.; Suomen Sokeri Oy), U.S. Pat. No. 5,084,104 (Heikkila, H. & Hyoky, G.; Cultor, Ltd.) and U.S. Pat. No. 4,168,988 (Riehm, T. & Hofenk, G; Institut voor Bewaring en Verwerking van Landbouwprodukten) and the publications referred to therein.
When xylose is prepared by hydrolyzation of xylan, an alternative to the above methods is to purify the xylan prior to its hydrolyzation into xylose to obtain a xylose solution of sufficient purity. Also this procedure is very complex and cumbersome, as will be apparent from Browning, B. L., Methods of Wood Chemistry, II, Interscience Publishers, New York, 1967, and Fry, S. C., The Growing Plant Cell Wall: Chemical and Metabolic Analysis, Longman Scientific & Technical, England, 1988.
In accordance with German Offenlegungsschrift 1,643,940 (Eickenmeyer, R. & Scholler, H.), crystalline xylose is recovered from a hydrolysate of pentosan- and cellulose-containing natural substances by crystallization from a syrup containing at least about 70% of xylose. The syrup is introduced into a crystallizer at 60-70.degree. C., and a crystal mass containing 15-33% of xylose on the amount of xylose supplied to the crystallizer is taken from the crystallizer at 48-52.degree. C. Crystals are separated from this crystal mass by centrifugation, and the mother liquor, the amount of which is 300-100% of the fresh syrup supplied to the system, is combined with the starting material hydrolysate. The resultant mixture of mother liquor and hydrolysate is treated in a cation exchanger and an anion exchanger, and after a subsequent decolouring treatment the mixture is evaporated in order to obtain a syrup to be supplied to the crystallizer. In addition to cumbersome purification treatments, the method thus comprises very extensive recycling. In accordance with this publication, the small amount of xylose obtained in one crystallization (the small yield as compared with the xylose supplied to the crystallizer) is due to fact that when the temperature falls below about 48.degree. C., the crystallization rate will be very small on account of the fact that the viscosity of the solution substantially increases when the temperature drops.
U.S. Pat. No. 3,981,739 to Dmitrovsky et al. relates to a method of continuously crystallizes sugars (sucrose, dextrose, fructose, lactose, carbohydrates). The method involves controlled growth of crystals in two-stage evaporative crystallization starting from seed crystals of small size. The crystals in the first stage are substantially larger than the seed crystals, and crystals of increased size are produced in the second stage.
U.S. Pat. No. 4,199,373 to Dwivedi et al. relates to a process for manufacturing free-flowing mixtures of fructose and glucose avoiding the disadvantages of earlier processes (such as the need of sophisticated machinery and careful control, high energy costs and low yield). The process is a solidification method; it includes no separation of crystals and mother liquor. A highly concentrated solution is seeded and permitted to stand (thus allowing crystallization to take place) at a specific temperature and relative humidity, recovered, dried and ground. Too low a concentration results in a pasty mass, too high a concentration results in a glassy mixture. It is essential that the surrounding air has a relative humidity below 50% and a temperature between 50-90.degree. F. (10-32.degree. C.) .
Other total solidification processes are disclosed, for example, in U.S. Pat. No. 4,297,146 to Mise et al.; U.S. Pat. No. 4,595,418 to Yoshino; and U.S. Pat. No. 4,640,717 to Shukla et al.
U.S. Pat. No. 4,634,472 to Niekamp et al. provides a process for manufacturing an enriched fructose syrup. In this process, a feed syrup (75-89% dry solids concentration) temperature which is appropriate for crystallization of glucose is established. It is well known in the art that the easy crystallization of glucose, even at low purity, is often a problem, as for instance in the case of honey (typical solid concentration 81-85%, about 40% of glucose and about 30% fructose on dry solids). It is also known (see Harold E. Horn, "Dextrose: An Alternative to Sucrose in Panned Confections", The Manufacturing Confectioner for 1977) that glucose crystallization is increasingly inhibited at viscosities of 10,000-100,000 cP (10-100 Pas). Calculated from Example 1 of U.S. Pat. No. 4,634,472, the crystallization viscosity is only about 2000 cP, which represents a very low viscosity solution. No water can be used as a diluent in the process according to U.S. Pat. No. 4,634,472 (Column 5, lines 20-25), since the crystals would dissolve.
U.S. Pat. No. 4,816,079 to Ahrens et al. relates to a process for continuous crystallization of dextrose monohydrate. The process is, in principle, a traditional cooling crystallization method based on crystal growth. Part of the feeding syrup is subjected to a shearing process for a period of 0.01-2 seconds to initiate nucleation for producing seed crystals for the process.
Accordingly, there is need for an economical and efficient process to achieve a high recovery of crystalline product from a solution containing same, especially a source, having lower levels of crystallizable product than are directly processable under comparable conditions to achieve the same yield.
It is then a principal object of the invention to achieve an improvement in overall yield of recovered crystallizable organic compounds from solutions containing same.
It is a further object to economically utilize in such processes material streams impure in crystallizable organic compounds contained therein.
It is also an object to employ run-off or recycle stream from industrial processes containing crystallizable organic compounds as a source for the efficient recovery of such compounds in good yield.